The present invention generally relates to an isolation cell and method for isolating output signals and, more particularly, to a low leakage isolation cell.
Isolation cells are used to isolate output signals of a powered down domain. Output signals of a powered down domain may have an intermediate voltage level, so isolation cells are used to clamp the signals to a specific voltage level before passing them to another voltage domain. FIG. 1 shows a circuit diagram of a conventional isolation cell 10. The isolation cell 10 includes a first inverter 12, a logic circuit 14, and a second inverter 16. The first inverter 12 receives an isolation control signal “iso” and generates an inverted version “isob” of the signal “iso”. The logic circuit 14, which is a NAND gate, receives an input signal “a” from a first power domain and the inverted solation control signal isob, and provides a logic output signal “sn1”. The second inverter 16 is connected to the logic circuit 14 to receive the logic output signal “sn1”, and generates an inverted version “x” of the output signal “sn1”, which may be provided to a second power domain.
From the circuit diagram of FIG. 1, a logic expression of the isolation cell 10 is: x=iso·a, and a truth table of the isolation cell 10 is:
IsoAX000011100110For normal operations, the first and second inverters 12 and 16, and the logic circuit 14 must be powered by the power of the second power domain (the “to” power domain), so that when the first power domain (the “from” power domain) is powered down, the isolation cell 10 can still operate normally.
Low leakage in stop or power down mode is a key feature of integrated circuits, and low leakage is desirable to improve battery life in portable devices. However, since the when the PMOS transistors of the first inverter 12, the logic gate 14 and the second inverter 14 are connected to the voltage supply of the second power domain (i.e., Vddc), the isolation cell 10 suffers from considerable leakage current.
It would be advantageous to have a low leakage isolation cell.